Water pollution mechanisms caused by bottom sediments in limnology and coastal waters and methods for improving water environments

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This research deals with mechanisms of water pollution caused by bottom sediments in various waters and examines methods for improving the water environments in these waters. Main results are summarized as follows.The effect of diffusional mass transfer between bottom sediment and overlying water were investigated theoretically and experimentally. The models to estimate the vertical diflusive flux at the sediment/water interface were formulated for smooth bed, rough bed and turbulent flow in a rectangular concave. Moreover, to investigate flow structures over such bed forms, the turbulent flows in the concave were measured by using a PIV and analyzed by employing a wavelet transform and a proper orthogonal decomposition.A plume generated from a sloping boundary of constant buoyancy was investigated experimentally and theoretically. A theoretical model was developed assuming a self-similar flow structure. Solutions for buoyancy and velocity fields were successfully obtained. The agreement between the analysis and experiments was quite satisfactory. The analysis enables us to quantitatively estimate water quality transport caused by the plume in a reservoir.To clarify the cycling of nutrient salts in river estuary, field observations were carried out in Osaka Bay area. Results show that 20% of the amounts ofTP and TN supplied from the upstream have settled in the lower reaches of the river, and that most of the organic substances have settled in the area of lOknroflshore from the river mouth. It has been understood that the denitrification phenomenon is playing an important role of a removal of nitrogen in the estuary.The characteristic of tidal current at Maiko beach was investigated through field data. The eastern direction flow due to the pressure gradient was prominent there and it had a major effect to sediment transport. Then, A numerical method to simulate a run-up over a sloping bed profile was proposed.